Differential Nitrogen Retention from Casein, Lactalbumin, and Soy Protein, and Hydrolysates Therefrom

Abstract

Summary The various data illustrating different methods for the biological evaluation of the protein lead, generally speaking, to the same conclusions. The evidence clearly shows the superiority of lactalbumin over casein and the superiority of both casein and lactalbumin over soy protein. If a relative value of 100 was assigned to the lactalbumin, the value for the casein would be about 80 and that of the soy protein between 50 and 60. These general relationships have been indicated by published data from time to time, although the magnitude of the differences have been subject to variations depending upon the particular methods used. Reference has been made to the linear relationship between nitrogen balance and absorbed nitrogen wherein variations in the slopes of the curves are of value as a means for determining the relative or absolute merits of the test materials. Such evidence shows that in each instance the original protein is slightly superior to its hydrolysate in respect to maintenance of a positive nitrogen balance. This relationship, however, is not consistently shown by the calculated biological values (table 1).The relatively greater difference between the iso-electric albumin and its hydrolysate may be accounted for by the intermediate step of peptization before hydrolysis. The difference between the peptized lactalbumin and the hydrolysate prepared direct therefrom is not of substantial magnitude. Two significant deductions may be drawn from the data as a whole. First, appropriate preparation of protein hydrolysates may offer a means of extracting unavailable or indigestible protein from sources not recognized as suitable food products. Second, the mere stipulation of a given percentage of protein in foods or feed stuffs is inadequate in describing their protein value. For instance, in using the nitrogen balance data as a basis of interpretation the soy protein has only about 50 per cent the value of the lactalbumin and about 60 per cent the value of casein; the other data are in substantial agreement. From the standpoint of protein conservation and food economy it is seemingly apparent that as a particular protein more nearly approaches the ideal a commensurate reduction in the amount of dietary protein could be made. The physiological aspects of the lower protein intake may also be of importance from the standpoint of health because the data indicate that a large proportion of the nitrogen absorbed from inferior protein is excreted without commensurately contributing to physiological functions. The average survival period of the groups of animals used in these investigations when furnished the nitrogen-free diet following the test period tends to indicate a significant effect of the previous protein intake. The average survival time for the lactalbumin group was about 30 per cent longer than for the soy protein group, and the survival time for the casein group was about 24 per cent longer than for the soy protein group. One may presume that the variations observed are due to an imbalance or deficiency of amino acids, or possibly to an absence of physiologically important linkages in the inferior protein molecule. If evidence should clearly disclose that the former condition is the cause of the differences, the fortification and enhancement of a deficient protein by appropriate synthetic amino acids in proper balance should prove to be of importance from the standpoint of health and economy and conservation of natural food protein.